Edward J. Tarbuck & Frederick K. Lutgens

 Alfred Wegener First proposed hypothesis, 1915 Published The Origin of Continents and Oceans  Continental drift hypothesis Supercontinent called Pangaea began breaking apart about 200 million years ago Continents "drifted" to present positions Continents "broke" through the ocean crust

 Wegener's continental drift hypothesis Evidence used by Wegener  Fit of South America and Africa  Fossils match across the seas  Rock types and structures match  Ancient climates Main objection to Wegener's proposal was its inability to provide a mechanism

 More encompassing than continental drift  Associated with Earth's rigid outer shell Called the lithosphere Consists of several plates  Plates are moving slowly  Largest plate is the Pacific plate  Plates are mostly beneath the ocean

 Asthenosphere Exists beneath the lithosphere Hotter and weaker than lithosphere Allows for motion of lithosphere  Plate boundaries All major interactions among plates occur along their boundaries

 Plate boundaries Types of plate boundaries  Divergent plate boundaries (constructive margins) Two plates move apart Mantle material upwells to create new seafloor Ocean ridges and seafloor spreading Oceanic ridges develop along well- developed boundaries Along ridges, seafloor spreading creates new seafloor

 Plate boundaries Types of plate boundaries  Divergent plate boundaries (constructive margins) Continental rifts form at spreading centers within a continent

 Plate boundaries Types of plate boundaries  Convergent plate boundaries (destructive margins) Plates collide, an ocean trench forms and lithosphere is subducted into the mantle  Convergent plate boundaries (destructive margins) Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere Pockets of magma develop and rise Continental volcanic arcs form Examples include the Andes, Cascades, and the Sierra Nevadan system

 Plate boundaries Types of plate boundaries  Convergent plate boundaries (destructive margins) Oceanic-oceanic convergence Two oceanic slabs converge and one descends beneath the other Often forms volcanoes on the ocean floor Volcanic island arcs forms as volcanoes emerge from the sea Examples include the Aleutian, Mariana, and Tonga islands

 Plate boundaries Types of plate boundaries  Convergent plate boundaries (destructive margins) Continental-continental convergence When subducting plates contain continental material, two continents collide Can produce new mountain ranges such as the Himalayas

 Plate boundaries Types of plate boundaries  Transform fault boundaries Plates slide past one another No new crust is created No crust is destroyed Transform faults Most join two segments of a mid-ocean ridge At the time of formation, they roughly parallel the direction of plate movement Aid the movement of oceanic crustal material

 Evidence for the plate tectonics model Paleomagnetism  Probably the most persuasive evidence  Ancient magnetism preserved in rocks  Paleomagnetic records show Polar wandering (evidence that continents moved) Earth's magnetic field reversals Recorded in rocks as they form at oceanic ridges

 Evidence for the plate tectonics model Earthquake patterns  Associated with plate boundaries  Deep-focus earthquakes along trenches provide a method for tracking the plate's descent Ocean drilling  Deep Sea Drilling Project (ship: Glomar Challenger )

 Evidence for the plate tectonics model Ocean drilling  Age of deepest sediments Youngest are near the ridges Older are at a distance from the ridge  Ocean basins are geologically young

 Evidence for the plate tectonics model Hot spots  Rising plumes of mantle material  Volcanoes can form over them e.g., Hawaiian Island chain Chains of volcanoes mark plate movement

 Measuring plate motion By using hot spot “tracks” like those of the Hawaiian Island - Emperor Seamount chain Using space-age technology to directly measure the relative motion of plates  Very Long Baseline Interferometry (VLBI)  Global Positioning System (GPS)

 Driving mechanism of plate tectonics No one model explains all facets of plate tectonics Earth's heat is the driving force Several models have been proposed  Slab-pull and slab-push model Descending oceanic crust pulls the plate Elevated ridge system pushes the plate

 Driving mechanism of plate tectonics Several models have been proposed  Plate-mantle convection Mantle plumes extend from mantle-core boundary and cause convection within the mantle Models Layering at 660 kilometers Whole-mantle convection Deep-layer model